Fix native browser ESM loading

[zuchka]

Summary

• make index.mjs a self-contained native ES module that works in browsers and URL-based module loaders
• preserve Node CommonJS/ESM function identity through a Node-specific ESM compatibility wrapper
• expose the explicit remove-markdown/index.mjs subpath with matching ESM declarations
• generate the portable ESM implementation from the canonical CommonJS source and fail tests if they drift
• document direct browser ESM usage and the supported CDN/MIME-type boundary

Problem

The 0.7.0-rc.0 ESM entry imported its default export from index.js. Package-aware Node and Deno npm resolution treated that file as CommonJS and worked, but native browser and remote URL loaders parsed it as ESM and failed with:

The requested module './index.js' does not provide an export named 'default'

This follow-up to #119 removes that dependency from the portable ESM entry while retaining the established CommonJS behavior and Node interop.

Compatibility design

• index.js remains the canonical, unchanged CommonJS implementation.
• index.mjs is generated as a self-contained ESM implementation.
• index.node.mjs remains a thin CommonJS interop wrapper for normal Node package imports.
• Browser-aware resolvers select index.mjs; Node selects index.node.mjs.
• The generation check runs during prepublishOnly and the module test suite.

Validation

• 39 existing behavior tests
• module and packed-tarball tests on Node 18, 20, 22, 24, and 26
• real-browser unbundled index.mjs import
• Deno 2.9.5 local, HTTP URL-module, and packed npm-package tests
• TypeScript 4.7, 5.9, and 7 NodeNext consumers
• Are the Types Wrong validation, including the explicit ESM-only entry
• esbuild and Rollup browser-condition selection and runtime checks
• pnpm 11.5, Yarn 4.18 Plug'n'Play, and Bun 1.4 consumers
• 22,330 CommonJS/ESM differential assertions against published 0.6.4
• successful npm publish dry run
• zero production audit findings

Rollout

0.7.0-rc.0 is already published and immutable. After this merges, the next prerelease should be published as 0.7.0-rc.1 under the next dist-tag and receive a short browser-focused soak before promotion.

Performance issue #118 remains separate.

[zuchka]

@tukkek lmk your thoughts